Traveling wave based relay protection

ABSTRACT

An apparatus includes at least one Rogowski coil and a processor. The at least one Rogowski coil is positioned within an electrical power distribution network to detect a first traveling wave current caused by a fault on an electrical power transmission line of the network, generate a first signal indicative of detection of the first traveling wave, detect a second traveling wave current caused by the fault on the transmission line, and generate a second signal indicative of detection of the second traveling wave. The processor is adapted to receive the first signal and the second signal and to determine, based on the first signal and the second signal, where on the transmission line the fault occurred.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/403,651, filed Mar. 13, 2009, which is a divisional application ofU.S. patent application Ser. No. 10/891,449, filed on Jul. 15, 2004. Thecontents of the prior applications are incorporated herein by referencein their entirety.

TECHNICAL FIELD

This description relates to fault protection, and more particularly totraveling wave based relay protection.

BACKGROUND

Power transmission lines can carry alternating current (AC). When afault occurs on a line, it is useful to rapidly determine the existenceand location of the fault, so that protective measures can be takenbefore components connected to the line are damaged. The location of thefault also may be used in fixing the cause of the fault.

SUMMARY

In one general aspect, an apparatus includes at least one Rogowski coiland a processor. The at least one Rogowski coil is positioned within anelectrical power distribution network to detect a first traveling wavecurrent caused by a fault on an electrical power transmission line ofthe network, generate a first signal indicative of detection of thefirst traveling wave, detect a second traveling wave current caused bythe fault on the transmission line, and generate a second signalindicative of detection of the second traveling wave. The processor isadapted to receive the first signal and the second signal and todetermine, based on the first signal and the second signal, where on thetransmission line the fault occurred.

Implementations may include one or more of the following features. Forexample, the apparatus can include a single Rogowski coil that generatesthe first and second signals, or a first Rogowski coil that generatesthe first signal and a second Rogowski coil that generates the secondsignal. The processor can be further operable to receive a timingsynchronization signal. When a bus electrically connected to thetransmission line, the second traveling wave current can be caused bythe fault on the electrical power transmission line and can be reflectedby the bus.

In another general aspect, an electrical protection apparatus includes afirst Rogowski coil, a second Rogowski coil, and a protection device.The first Rogowski coil is positioned to detect a first traveling wavecurrent on a first transmission line of a power distribution network andto generate a first signal indicative of a polarity of the firsttraveling wave caused by a fault within the network. The second Rogowskicoil is positioned to detect a second traveling wave current on a secondtransmission line of the network and to generate a second signalindicative of a polarity of the second traveling wave caused by thefault within the network. The protection device is adapted to receivethe first signal and the second signal and is operable to determine,based on the first signal and the second signal, where in the networkthe fault occurred.

Implementations may include one or more of the following features. Forexample, the protection device can include a relay and a processor. Theapparatus can further include a circuit breaker operable to open inresponse to a signal from the protection device, where the signal isgenerated by the protection device upon the determination by theprotection device of where the fault in the network occurred. Theapparatus can further include a first circuit breaker positioned on thefirst transmission line and a second circuit breaker positioned on thefirst transmission line, where the protection device is further operableto cause the first circuit breaker, the second circuit breaker, or bothcircuit breakers to open in response to a determination by theprotection device of where the fault in the network occurred. Theapparatus can further include a busbar, to which the first transmissionline and the second transmission line are electrically connected.

In another general aspect, determining the location of a fault on anelectrical power transmission line includes receiving a first signalfrom a Rogowski coil positioned to detect a first traveling wave currentcaused by the fault, where the first signal is indicative of a time atwhich the first traveling wave is detected, receiving a second signalfrom a Rogowski coil positioned to detect a second traveling wavecurrent caused by the fault, where the second signal is indicative of atime at which the second traveling wave is detected, and determining,based on the first signal and the second signal, where on thetransmission line the fault occurred.

Implementations may include one or more of the following features. Forexample, the first signal and the second signal can be received from thesame Rogowski coil, or the first signal can be received from a firstRogowski coil and the second signal can be received from a secondRogowski coil. A timing synchronization signal may be received, and,based on the timing synchronization signal, the first signal, and thesecond signal, a determination may be made as to where on the line thefault occurred.

The transmission line can include a bus electrically connected to thetransmission line. At least one of the Rogowski coils can be adapted fordetecting a traveling wave current caused by a fault on the electricalpower transmission line that is reflected by the bus and can be adaptedfor generating a third timing signal indicative of a time at which thereflected traveling wave is detected.

In another general aspect, protecting a power apparatus from a fault ina power distribution network includes receiving a first signal from afirst Rogowski coil positioned to detect a first traveling wave currenton a first transmission line of the network, where the first signal isindicative of a polarity of the first traveling wave; and receiving asecond signal from a second Rogowski coil positioned to detect a secondtraveling wave current on a second transmission line of the network,where the second signal is indicative of a polarity of the secondtraveling wave. The location of the fault in the network is determinedbased on the first signal and the second signal, and a current flow on atransmission line of the network is halted based on the determination ofthe fault location. The transmission line upon which the current flow ishalted can be the first or second transmission line, or can be atransmission line in the network other than the first transmission lineor the second transmission line. A first tracking pulse having apredetermined amplitude and width may be generated in response to afirst detected traveling wave current that exceeds a predeterminedthreshold value, and a second tracking pulse having a predeterminedamplitude and width can be generated in response to a second detectedtraveling wave current that exceeds a predetermined threshold value.Based on the first tracking pulse and the second tracking pulse, adetermination may be made as to where the fault in the network occurred.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of the temporal relations between the generation oftraveling waves on a transmission line at a fault location andreflections of the traveling waves at points of impedance changes on theline.

FIG. 2 is a graph of instantaneous current on a transmission lineshortly before and shortly after a fault in the line occurs.

FIG. 3 is a schematic view of a transmission line and two Rogowski coilsused to measure current changes on the line.

FIGS. 4 and 5 are graphs of the instantaneous current and theinstantaneous change in current on a transmission line on opposite sidesof a fault in the line shortly before and shortly after the faultoccurs.

FIG. 6 is a schematic diagram of a power network protection system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A fault in a power system causes traveling waves (TWs) that propagatethrough the system away from the fault location at velocities close tothe speed of light. The TWs reflect at points where the impedance of thesystem changes. TWs can totally reflect, or can partially reflect andpartially refract, with the refracted portion continuing to travel inthe same direction. TWs have a fast rising front and a slower decayingtail, and have magnitudes that decrease with time. When TWs aregenerated, then both traveling wave voltages (TWVs) and traveling wavecurrents (TWCs) exist. TWCs can be used for fast relay protection andaccurate determination of fault locations in power systems.

The surge impedance Z_(s) of a transmission line is given byZ _(s) =√{square root over (L/C)},  (1)where L is the line inductance in Henries per unit length, and C is theline capacitance in Farads per unit length. Faults in power lines causetraveling waves that propagate along the line away from the fault. Thevelocity of a traveling wave, C_(TW), is given byC _(TW)=1/√{square root over (LC)}  (2)and is approximately equal to the speed of light for most transmissionlines. The traveling wave emitted from a fault has a traveling wavevoltage (TWV or V_(TW)) and a traveling wave current (TWC or i_(TW)). Atthe instant of the fault, the TWV and the TWC are related byV _(TW) =Z _(s) ×i _(TW),  (3)where V_(TW) is the instantaneous voltage on the line, and i_(TW) is theinstantaneous current in the line.

After a traveling wave is emitted from the fault location, the wavepropagates along the line until the wave reaches a point on the linewhere the impedance changes (e.g., a transformer or a bus). Because ofthe impedance change, the wave is reflected back along the line or ispartially reflected and partially transmitted. The time at whichindividual reflections and transmissions occur on the line can be usedto determine the location of the fault on the line.

For example, as shown in FIG. 1, a transmission line 102 is connected toa first bus 104, to a second bus 106, and to two sources 108 and 110.When a fault occurs at a location 112 on the line, traveling wavespropagate away from the fault location 112 on the line 102 towards thebuses 104 and 106 at a speed given by equation (2). When a travelingwave reaches a point of changing impedance on the line 102 (e.g., a bus104 or 106), the wave is either reflected or partially reflected andpartially transmitted. The amplitude, v_(r), of the reflected wave is

$\begin{matrix}{{v_{r} = {\frac{Z_{b} - Z_{s}}{Z_{b} + Z_{s}}v_{i}}},} & (4)\end{matrix}$where v_(i) is the amplitude of the incident traveling wave, Z_(b) isthe impedance of the bus 104 or 106, and Z_(s), as noted above, is thesurge impedance. The amplitude, v_(t), of the transmitted wave is

$\begin{matrix}{v_{t} = {\frac{2Z_{b}}{Z_{b} + Z_{s}}{v_{i}.}}} & (5)\end{matrix}$When the impedance of the bus 104 or 106 is much smaller than the surgeimpedance (Z_(b)<<Z_(s)), the reflected wave amplitude may beapproximated as being equal to the incident wave amplitude(|v_(r)|=|v_(i)|), and the transmitted wave amplitude may beapproximated as being zero (v_(t)=0).

Because the ionized fault resistance at fault location 112 is usuallymuch less than the surge impedance Zs, the traveling wave that reflectsoff bus 104 is totally reflected at fault location 112 and travels backto bus 104, with a reversal of the polarity of the pulse. If the faultresistance has a higher value that is comparable to the value of thesurge resistance, the TWC will not totally reflect at the fault location112, and, instead, will also be partially transmitted through the faultlocation 112 toward the bus 106 with a reduced magnitude as given inequation (5). The energy of the first incoming traveling wave to arriveat bus 104 is significantly larger than the energy of the wave thatarrives at bus 104 after being reflected from bus 106 and transmittedthrough fault location 112. This enables reliable differentiationbetween the reflected and transmitted waves. Similarly, the TWC thatreflects off bus 106 will be partially reflected and partiallytransmitted by the ionized fault resistance at fault location 112.

The arrival times of the TWCs at buses 104 and 106 are shown in thelower portion of FIG. 1, where distance is measured on the horizontalaxis and time is measured on the vertical axis. For example, the TWCemitted from fault location 112 at time t=t_(o) in the direction of bus104 (i.e., the first direct wave) travels at speed C_(TW)=1/√{squareroot over (LC)} over a distance A and arrives at bus 104 at timet_(A1)=t_(o)+Δt_(A). The speed, distance, and time are related byΔt_(A)=A√{square root over (LC)}. The first TWC that is reflected frombus 106 and transmitted through fault location 112 arrives at bus 104 attime t_(A2), which is defined as:t _(A2) =t _(o) +Δt _(A)+2Δt _(B) =t _(o)+(A+2B)√{square root over(LC)},where B is the distance from the fault location 112 to bus 106. Thefirst TWC that is reflected from bus 104 and from fault location 112arrives at bus 104 at time t_(A3), which is defined as:t _(A3) =t _(o)+3Δt _(A) =t _(o)+3A√{square root over (LC)}.

The second TWC that is reflected from bus 106 and transmitted throughfault location 112 arrives at bus 104 at time t_(A4), which is definedas:t _(A4) =t _(o) +Δt _(A)+4Δt _(B) =t _(o)+(A+4B)√{square root over(LC)}.

The initial TWC emitted from fault location 112 arrives at bus 106 attime t_(B1), which is defined as:t _(B1) =t _(o) +Δt _(B)+2Δt _(B) =t _(o) +B√{square root over (LC)}.

The first TWC that is reflected from bus 106 and from fault location 112arrives at bus 106 at time t_(B2), which is defined as:t _(B2) =t _(o)+3Δt _(B) =t _(o)+3B√{square root over (LC)}.

The first TWC that is reflected from bus 104 and transmitted throughfault location 112 arrives at bus 106 at time t_(B3), which is definedas:t _(B3) =t _(o)+2Δt _(A) +Δt ₃ =t _(o)+(2A+B)√{square root over (LC)}.

The second TWC that is reflected from bus 106 and from fault location112 arrives at bus 106 at time t_(B4), which is defined as:t _(B4) =t _(o)+5Δt _(B) =t _(o)+5B√{square root over (LC)}.

FIG. 2 shows current waveforms recorded at buses 104 and 106 due to afault at location 112 on an 80-mile long transmission line 102 (as shownin FIG. 1). The arrival of TWCs at bus 104 (referred to as “Bus A” inFIG. 2) and bus 106 (referred to as “Bus B” in FIG. 2) is also evidentin the current waveforms shown in FIG. 2. The fault simulated in FIG. 2is a single-phase-to-ground fault in the A-phase of the AC transmissionline that occurs at t_(o)=0.5 ms at a distance of 50 miles from bus 104near the voltage peak (at 500 μs, 79°) of an AC cycle. For simplicity,FIG. 2 shows only the currents measured on the A-phase line and omitsrepresentation of the currents on the B-phase line and the C-phase line.

The lower trace of FIG. 2 shows the current at bus 106. The firstincoming TWC pulse reaches bus 106 at t_(B1)=660 μs and produces anapproximately 218 A positive step change in the line current. The firstTWC pulse to be reflected by bus 106 and by fault location 112 reachesbus 106 at t_(B2)=980 μs. The first TWC pulse that is reflected from bus104 and transmitted through fault location 112 arrives at bus 106 att_(B3)=1199 μs and produces an approximately 75 A negative step changein the line current.

The third TWC pulse reflected from the fault location 112 reaches bus106 at t_(B4)=1300 μs and produces an approximately 43 A positive stepchange in the line current.

The upper trace of FIG. 2 shows the current at bus 104. The firstincoming TWC pulse reaches bus 104 at t_(A1)=767 μs, and produces anapproximately 206 A positive step change in the line current. The firstTWC pulse to be reflected by bus 106 and transmitted through faultlocation 112 reaches bus 106 at t_(A2)=1090 μs and produces anapproximately 88 A negative step change in line current. The second TWCpulse to arrive at bus 104 after being reflected from bus 104 andreflected by the fault location 112 produces an approximately 77 Apositive step change in line current and reaches bus 104 at t_(A3)=1300μs. The second TWC pulse to be reflected by bus 106 and transmittedthrough fault location 112 reaches bus 106 at t_(A4)=1413 μs andproduces an approximately 23 A negative step change in line current.

The fault location 112 on the line 102 can be determined by measuringthe time difference between the time at which the initial pulse isreceived at bus 104 and the time at which the second pulse, which isreflected from the bus 104 and then from the fault, is received at bus104. The distance, D, from the bus 104 to the fault location 112 isgiven by:

$\begin{matrix}{D = {\frac{c_{TW} \times ( {t_{A\; 3} - t_{A\; 1}} )}{2}.}} & (6)\end{matrix}$

In the similar way, the fault location 112 on the line 102 can bedetermined by measuring the time difference between the time at whichthe initial pulse is received at bus 106 and the time at which thesecond pulse, which is reflected from the bus 106 and then from thefault, is received at bus 106. The distance, D, from the bus 106 to thefault location 112 is given by:

$\begin{matrix}{D = {\frac{c_{TW} \times ( {t_{B\; 2} - t_{B\; 1}} )}{2}.}} & (7)\end{matrix}$

The fault location can also be determined using synchronizedmeasurements of the arrival times of pulses that reach buses 104 and 106at the two ends of the transmission line 102. The times can besynchronized by a GPS reference time signal available at each bus. Usingarrival times of pulses at the two ends of the line 102, the distance,D, from the bus 104 to the fault location 112 is given by

$\begin{matrix}{{D = \frac{l - {c_{TW} \times ( {t_{A\; 1} - t_{B\; 1}} )}}{2}},} & (8)\end{matrix}$where l is the line length.

To reliably detect and time tag the arrival of a traveling wave, the TWCpulse must be filtered out from the current component corresponding tothe fundamental power frequency (e.g., 60 Hz). The arrival of a TWC at alocation on the line 102 (e.g., at bus 104 or 106) can be detected by acoil positioned on the transmission line 102 just before the bus 104 or106. The coil can be, for example, a Rogowski coil. Generally speaking,a Rogowski coil includes a conductive element that is wound around anon-magnetic core. The conductive element may be, for example, a metalwire or a metal deposit. The non-magnetic core may be made of anymaterial that has a magnetic permeability that is substantially equal tothe permeability of free space, such as, for example, an air core or aprinted circuit board (PCB) on which the conductive element is traced.

The output voltage of a Rogowski coil is proportional to the rate ofchange of measured current (di/dt) enclosed by the coil. Thus, Rogowskicoils are particularly sensitive to high-frequency components, and areable to amplify high-frequency signal components without using specialfilters. This unique feature of Rogowski coils makes them particularlysuitable for measuring rapid current changes and for detecting TWCs.

As shown in FIG. 3, a transmission line 302 is connected to powersources 304 and 306 and to buses 308 and 310. Rogowski coils 320 and 322are located on the transmission line 302 in close proximity to the buses308 and 310, respectively. The coils 320 and 322 can be constructedaccording to various techniques. Examples of such techniques arediscussed in, for example, U.S. Pat. No. 6,313,623, titled “HighPrecision Rogowski Coil,” and U.S. Pat. No. 6,680,608, titled “MeasuringCurrent Through An Electrical Conductor,” both of which are incorporatedby reference. For example, the coils 320 and 322 can include two or morearms that form a main loop (or loops) of the coils 320 and 322 whencoupled together. Various winding techniques for winding the conductiveelement may be used in constructing the coils 320 and 322, and the coils320 and 322 can include multiple coils that are associated with oneanother in various ways. These and other construction details related tothe coils 320 and 322 may be selected so as to ensure high levels ofsensitivity and accuracy in determining the current changes on thetransmission line 302.

The output signal from the Rogowski coils 320 and 322, which can belocated at a high voltage potential near the transmission line 302, canbe passed to electrical-to-optical converters 324 and 326, respectively,and then transmitted by optical fibers 328 and 330 tooptical-to-electrical converters 332 and 334, respectively, which can belocated close to electrical ground. Once the Rogowski coil signals havebeen re-converted to electrical signals, they can be further processedby processors 336 and 338. Processors 336 and 338 can communicate witheach other through a data transmission line 340 to compare the signalsthat they receive and generate.

Although the communications line 340 may communicate information betweenthe processors 333 and 338, there may nonetheless be some amount ofdelay in transmitting the various signals. When comparing currentsignals from the coils 308 and 310, relative timing information for thecurrent signals may be required in order to account for this delay (aswell as other delays that may occur) so as to make a meaningfulcomparison of the current signals.

Such timing information can be obtained from various sources. Forexample, an external synchronizing network 342 (e.g., a network thatprovides a GPS clock) may be set up to provide timing information.Processors 336 and 338 can receive timing information from thesynchronizing network, so that the arrival times of TWCs at Rogowskicoils 320 and 322 can be compared to an absolute reference standard. Asanother example, the processors 336 and 338 may time stamp theirrespective current measurements before transmission of the measurements.

FIG. 4 shows an instantaneous current measurement 400 at a bus 308 atone end of an 80-mile long transmission line 302 along with the changein current 405 measured by the Rogowski coil 320 located close to thebus 308. FIG. 5 shows the instantaneous current measurement 500 at a bus310 at the other end of the transmission line 302 and the change incurrent measured by the Rogowski coil 322 located close to the bus 310.When a fault occurs at a time t=500 μs, high-frequency transients aresuperimposed on the 60 Hz fundamental frequency in the current traces.The Rogowski coils 320 and 322 detect the changes in current andregister signals that are proportional to the temporal derivative of thecurrent. When the output signal of a Rogowski coil 320 or 322 exceeds athreshold value, processor 336 or 338, respectively, generates astandard amplitude and width tracking pulse that can be used by timinglogic within the processor 336 or 338 to determine the location of thefault on the transmission line according to equations (6) (7), or (8).

To achieve reliable fault detection, an instantaneous current leveldetector (ICLD), which can be implemented in hardware or software,provides a supervisory function by monitoring the instantaneous value ofthe current. Whenever a TWC having an amplitude within predeterminedthreshold values is detected by a processor 336 or 338, the processorgenerates a standard amplitude and width tracking pulse. A relay that isoperated in response to the detection of a TWC will not issue a tripcommand until the ICLD asserts and latches.

As discussed above, Rogowski coils may be used as the current sensingcoils of FIGS. 3-5. Rogowski coils are very sensitive to even low-levelcurrent changes, and, thus, are capable of, for example, detecting andinitiating clearing of sustained arcing fault currents. Such faultcurrents generally are at a small fraction of the maximum availablefault current, and are not much higher than the load currentsthemselves.

The ability to detect small current changes means that fault detectionlevels may be set relatively low, thereby reducing stress on (or damageto) equipment and speeding fault response times without sacrificingreliability. Moreover, a risk of fire propagation is reduced, and fasterresponse times (including a faster restoration of service) may beprovided.

Rogowski coils do not saturate, and, therefore, are capable of handlinglarge currents and avoiding false tripping of circuit breakers inresponse to faults outside the protected zone. The ability of aparticular Rogowski coil to avoid saturation may allow a single Rogowskicoil to provide current measurements over a wide measurement range, suchas, for example, from several amps to several hundred thousand amps. Asa result, such coils may be used to measure currents having a large DCcomponent. Also, Rogowski coils may operate over a wide frequency range,such as, for example, from approximately 0.1 Hz to over 1 MHz. Rogowskicoils also may be designed to provide a bandpass frequency response ofup to approximately 200 MHz or more. Additionally, Rogowski coils aregenerally immune to external magnetic fields, and, therefore, may avoidany effects of such fields on current measurements. Moreover, Rogowskicoils are relatively inexpensive and typically do not requiresubstantial space or wiring. Finally, a Rogowski coil is easilyinstalled by, for example, simply placing the relevant conductor throughthe coil (or by placing the coil around the conductor).

Because Rogowski coils are sensitive to changes in current, they can beused to detect a fault in less than one full 60 Hz cycle (i.e., 16.67ms). As is evident from the example described above, because TWCsproduced by a fault 112 propagate on the transmission line 102 at closeto the speed of light, and Rogowski coils can detect a TWC withsub-millisecond accuracy, a Rogowski coil can detect a TWC indicating afault in far less time than the time period of a 60 Hz cycle (i.e.,16.67 ms). Thus, one or more Rogowski coils can quickly identify a faultin a power transmission system and respond to the fault to protect thesystem (as explained in further detail below).

A differential busbar protection system using Rogowski coils isillustrated with reference to FIG. 6. As shown, an electrical powersystem includes a busbar 602 electrically connected to a first powersystem 604 (e.g., a source or a load) by a first transmission line 614and to a second power system 606 (e.g., a source or a load) by a secondtransmission line 616 and a third transmission line 618. Thus, currentcan flow into the busbar 602 on line 614 and out of busbar 602 on lines616 and 618. During normal operation, the sum of currents flowing intothe busbar 602 is equal to the sum of currents flowing out of the busbar602.

Rogowski coils 624, 626, and 628 are located close to busbar 602 andsense current and current changes in lines 614, 616, and 618,respectively. A relay 630 electrically connected to the Rogowski coils624, 626, and 628 and to circuit breakers 644, 646, and 648 located onrespective lines 614, 616, and 618 serves to provide integratedprotection against short circuits and other system malfunctions and/orfailures, as described in more detail below. As such, the relay 630 maybe programmed or otherwise associated with a predetermined algorithm forautomatically implementing the integrated protection scheme. Forexample, the relay 630 can include an ICLD to monitor the instantaneouslevel of the current and to generate a standard amplitude and widthtracking pulse whenever a TWC having an amplitude within predeterminedthreshold values is detected by Rogowski coils 624, 626, and 628. Thetracking pulses can be used to make logical decisions for protecting thenetwork (as explained in more detail below). Although only one relay 630is shown, two or more relays in communication with each other can beused in the system. For example, a separate relay can be associated withan individual Rogowski coil 624, 626, or 628.

With regard to the protection system 600, the relay 630 is capable ofproviding multiple types of protection against electrical or mechanicalmalfunctions and failures, and of integrating these types of protectioninto a cohesive protection scheme. Moreover, the relay 630 is capable ofinteracting with other relays and/or other coils in order to providefurther options for constructing an integrated electrical protectionsystem.

One type of protection afforded by the relay 630 is differentialprotection. In a differential protection scheme, the relay 630 operatesto compare the currents on lines 614, 616, and 618 to check if thecurrents have some predetermined relationship to one another. As oneexample, when a fault occurs at busbar 602 or otherwise between coils624, 626, and 628, a transient current pulse is created on each of thelines 614, 616, and 618. Because the fault occurs within the zonebetween coils 624, 626, and 628, the transient pulses on each of thelines has the same polarity. In such a case, the relay 630 can determinethat the fault is located within the zone and can trip each circuitbreaker 644, 646, and 648 to protect the busbar 602 from overload due topower supplied from power system 604 or 606.

As another example, when a fault occurs outside the busbar 602, such as,for example, on transmission line 616, the polarity of the transientpulse moving toward the busbar 602 (e.g., on line 616) will be oppositeto the polarity of the transient pulse moving away from the busbar 602(e.g., on lines 614 and 618). In such a case, the relay 630 candetermine that the fault is located on line 616 because the polarity ofthe pulse detected by Rogowski coil 626 is different from the polarityof the pulses detected by coils 624 and 628. Thus, relay 630 can tripcircuit breaker 646 to protect the busbar 602 from the fault on line 616but can allow power to continue to flow to/from power system 604 tobusbar 602 on line 614 and to/from power system 606 on line 618.Similarly, when a fault occurs on line 618, relay 630 can determine thatthe polarity of the pulse measured by coil 628 is different from thepolarity measured by coils 624 and 626 and, in response, can tripcircuit breaker 648 while leaving breakers 644 and 646 closed.

As a further example, when a fault occurs upstream of coil 624, thepolarity of the pulse measured by coil 624 is different from thepolarity measured by coils 626 and 628. Based on this information, relay630 can determine the location of the fault but need not trip any of thecircuit breakers.

The relay 630 may be, for example, a microprocessor-controlled,multi-function relay, such as a three-phase relay having multiplevoltage and/or current inputs. As discussed above, the relay 630 may bein communication with circuit breakers 644, 646, and 648, and 648,companion relays (not shown), control equipment (not shown), and othercircuit elements. For example, the relay 630 may be connected to anetwork switch/hub that supports having the relay 630 communicate withother relays in implementing an electrical protection system.

Using these and related techniques, electrical equipment may beprotected from damage due to fault currents. Moreover, by placing thecoils 624, 626, and 628 around selected pieces of circuitry/equipment,and thereby establishing the protection zones, a location as well as anexistence of a fault current may be accurately detected. Additionally, anumber of current sensors (coils) and relays may be minimized (relativeto other electrical protection systems) so as to increase an ease ofinstallation.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A system comprising: a first Rogowski coilpositioned on a first transmission line of an AC electrical powernetwork, the first Rogowski coil configured to detect a first travelingwave current caused by a fault in the network and to generate a firstsignal indicative of a polarity of the first traveling wave; a secondRogowski coil positioned on a second transmission line of the ACelectrical power network, the second Rogowski coil configured to detecta second traveling wave current caused by the fault in the network andto generate a second signal indicative of a polarity of the secondtraveling wave; and a protection device configured to: receive the firstsignal and the second signal, and determine where in the network thefault occurred, based on the first signal indicative of the polarity ofthe first traveling wave and the second signal indicative of thepolarity of the second traveling wave.
 2. The system of claim 1, furthercomprising a busbar electrically connected to the first transmissionline and to the second transmission line.
 3. The system of claim 2,further comprising: a third Rogowski coil positioned on a thirdtransmission line of the AC electrical power network, the third Rogowskicoil configured to detect a third traveling wave current caused by thefault in the network and to generate a third signal indicative of apolarity of the third traveling wave, and wherein the protection deviceis further configured to receive the third signal.
 4. The system ofclaim 3, wherein the busbar is electrically connected to a first powersystem through the first transmission line, and the busbar iselectrically connected to a second power system through the secondtransmission line and the third transmission line.
 5. The system ofclaim 4, wherein the first power system comprises a source, and thesecond power system comprises a load.
 6. The system of claim 3, whereinthe protection device is configured to determine where in the networkthe fault occurred based on the first signal indicative of the polarityof the first traveling wave, the second signal indicative of thepolarity of the second traveling wave, and the third signal indicativeof the polarity of the third traveling wave.
 7. The system of claim 3,wherein the protection device comprises a relay.
 8. The system of claim7, wherein the relay is configured to determine where in the network thefault occurred by comparing the polarity indicated by each of the firstsignal, the second signal, and the third signal.
 9. The system of claim8, wherein if the polarity indicated by each of the first signal, thesecond signal, and the third signal is the same, the relay determinesthat the fault occurred within a region between the first Rogowski coil,the second Rogowski coil, and the third Rogowski coil.
 10. The system ofclaim 9, wherein the relay determines that the fault occurred on thebusbar.
 11. The system of claim 8, wherein if the polarity indicated bythe first signal is different from the polarity indicated by each of thesecond signal and the third signal, the relay determines that the faultoccurred within a region outside of the first Rogowski coil, the secondRogowski coil, and the third Rogowski coil.
 12. The system of claim 11,wherein the relay further determines that the fault occurred on thefirst transmission line.
 13. The system of claim 12, wherein the relayis further configured to cause current to stop flowing in the firsttransmission line in response to the determination that the faultoccurred on the first transmission line.
 14. The system of claim 3,further comprising: a first circuit breaker located on the firsttransmission line; a second circuit breaker located on the secondtransmission line; and a third circuit breaker located on the thirdtransmission line, wherein the first circuit breaker, the second circuitbreaker, and the third circuit breaker are in communication with theprotection device.
 15. The system of claim 2, wherein the busbar iselectrically connected to a first power system through the firsttransmission line and electrically connected to a second power systemthrough the second transmission line.
 16. The system of claim 1, whereinthe protection device comprises a relay.
 17. The system of claim 1,further comprising: a first circuit breaker located on the firsttransmission line; and a second circuit breaker located on the secondtransmission line, wherein the first circuit breaker and the secondcircuit breaker are in communication with the protection device.
 18. Thesystem of claim 1, wherein the protection device comprises a first relayassociated with the first Rogowski coil, and a second relay associatedwith the second Rogowski coil.